1. Field of the Invention
The present invention relates to a polyester film, a method for manufacturing the same, and a solar cell module.
2. Description of the Related Art
In recent years, photovoltaic power generation in which sunlight is converted into electricity has drawn attention from the viewpoint of protection of the global environment. A solar cell module for use in the photovoltaic power generation generally has a structure in which (Sealing material)/Solar cell device/Sealing material/Back sheet are disposed in this order on a glass substrate through which sunlight enters.
The solar cell module is required to have high weather resistance so as to maintain the cell performance, such as power generation efficiency, over a long period of time, e.g., several decades, even under severe use environments in which the solar cell module is exposed to wind, rain and direct sunlight. In order to give such weather resistance, various materials, such as a back sheet or a sealing material that seals a device, constituting the solar cell module, are also required to have excellent weather resistance. Moreover, strong weather resistance is required also in the adhesion or the like between these materials, such as between the back sheet and the sealing material (e.g., ethylene-vinyl acetate copolymer (EVA))).
For the back sheet for constituting the solar cell module, resin materials, such as polyester, are generally used. The polyester generally has many carboxyl groups or hydroxyl groups on the surface, and thus is likely to be hydrolyzed in an environment where moisture is present, and tends to remarkably deteriorate with time. Therefore, polyester for use in the solar cell module placed in an environment in which the solar cell module is always exposed to wind and rain, such as outdoors, is required to suppress the hydrolysis properties. However, when reduction in the acid value is tried in order to suppress the hydrolysis properties, the amount of carboxyl groups or hydroxyl groups on the surface of a film is difficult to control in a desired range, e.g., the amount becomes excessively low, which, on the contrary, results in the fact that the adhesion with an adherend is impaired.
As a technique relating to the above description, a technique for suppressing delamination (separation between layers) by adjusting the X ray diffraction intensity ratio (plane orientation) of polyester in a specific range in order to prevent poor adhesion (separation) due to cohesion failure inside a PET film, for example, is disclosed as a method for improving the adhesion when using polyester (e.g., Japanese Patent Application Laid-Open (JP-A) No. 2007-268710).
In addition, a film for sealing the back surface of a solar cell in which a thermal adhesive layer is disposed on a polyester film is disclosed (e.g., JP-A No. 2003-60218).
However, in the polyester film exhibiting a specific X ray diffraction intensity ratio (plane orientation), the hydrolysis of the PET surface due to the elapse of a long period cannot be suppressed, and thus the molecular weight reduces, which results in the fact that the surface embrittles and the adhesion breaks. Similarly also in the film on which the thermal adhesive layer is disposed, the surface embrittles with time and the decomposition with time of the thermal adhesive layer proceeds, and thus, the adhesion reduces.
As described above, in the conventional techniques, the fact is that the adhesion after the elapse of time is still insufficient due to the influences of the hydrolysis properties with respect to the weather resistance over a long period of time, and a further improvement has been desired in the weather resistance over a long period of time.